Since priorly, various types of semiconductor chips have been obtained by cutting off a plurality of element areas formed and arrayed on a semiconductor wafer at boundary positions. Recently, the cutting of semiconductor wafers is carried out by, for example, cleaving the wafers along scribing marks with the scribing marks provided at boundary positions of the element formed areas at the end edge on the surface of the semiconductor wafers.
FIG. 11 and FIG. 12 show a conventionally general method for cleaving semiconductor wafers. The cleaving method is called a "three-point bending method." First, a plurality of scribing marks 2 are arrayed and formed at the end edge on the surface of a semiconductor wafer 1 (These scribing marks 2 are provided at the boundary positions of the element formed areas on a semiconductor wafer 1). Next, a pair of fulcrum members 4a and 4b are disposed in parallel to the scribing marks 2 with each of the scribing marks 2 placed therebetween on the surface 3 where the scribing marks are formed. Also, on the rear side 5 opposite the scribing mark formed surface, a fulcrum member 6 is disposed so as to become parallel to each of the scribing marks 2, at the position opposed to the above scribing marks 2. After that, a tensile force is given to the scribing marks 2 by causing fulcrum forces P2, P1, and F1 to operate thereon from these fulcrum members 4a, 4b and 6, whereby the semiconductor wafer 1 is cleaved at the plane ZOY from the position of the scribing marks 2.
The relationship between a shearing force and a bending moment in carrying out a cleaving by the "three-point bending method" is expressed as shown in FIG. 13. As shown in FIG. 13, the positive or negative polarity of the shearing force is reversed at the lower fulcrum member 6 used as the boundary. In fact, the operating point of a fulcrum force F1 acting from the fulcrum member 6 on the semiconductor wafer 1 has a definite width although being slight. Therefore, the shearing stress does not become zero in the entirety of the operating area of the fulcrum force F1. As shown in FIG. 12, a portion of a minute cubic volume dva of one-sided half of the scribing mark is taken for instance, wherein a shearing stress .tau. parallel to the plane ZOY to be cleaved, and a bending tensile stress .sigma..sub.ax perpendicular to the plane ZOY to be cleaved are produced.
If the shearing stress and bending tensile force are expressed in terms of Mohr's stress circle, they will become as shown in FIG. 14. That is, the direction of action of the ruling maximum main stress .sigma..sub.a1 of cleavage produced on the cleavage plane of the scribing mark is not a direction .sigma..sub.ax perpendicular to the vertical cleavage plane (plane ZOY) ideal to the semiconductor wafer 1, but has an angle .alpha.1 from the direction .sigma..sub.ax, (which can be easily calculated by Mohr's stress circle). Therefore, as in the cleavage plane of a cleavage state illustrated in FIG. 15, an oblique cleaved portion 7a is produced at the portion at the lower side of the scribing mark 2 on the cleavage plane 7 of the semiconductor wafer 1, whereby such a problem arises, by which an expected cleavage plane could not reliably be obtained.
In addition, as shown in FIG. 16, when actually carrying out a cleaving work, the fulcrum member 6 does not become parallel to the scribing marks 2, whereby a possibility exists, that the fulcrum member 6 is disposed so as to cross the extension line of the scribing marks 2. In such cases, a problem arises, by which a cleavage plane 7 bent as shown in FIG. 17 would be produced.
The present invention was developed in order to solve the above shortcomings and problems, and it is therefore an object of the invention to make the shearing stress zero in the vicinity of the scribing marks, and to make the direction of the maximum main stress produced in the scribing mark perpendicular to the ideal vertical cleavage direction. That is, the invention is to provide a semiconductor wafer cleaving method and apparatus which enables an ideal mirror-finished cleavage plane by operating only the bending tensile stress onto scribing marks.